NEW YORK – Nielsen, the dominant source of TV ratings, on Tuesday said it had agreed to buy Arbitron for about $1.26 billion to expand into radio measurement.

Arbitron pays 70,000 people to carry around gadgets that register what stations they’re listening to. Since Nielsen also collects cash register data, CEO David Calhoun said buying Arbitron will let Nielsen be a one-stop shop for advertisers who want to know how the radio advertising they buy affects product sales.

The acquisition will let Nielsen expand the amount of media consumption it tracks from about 2 hours per person per day to 5 hours, Calhoun said.

“You don’t find many mediums that allow for that kind of increase,” Calhoun said.

Arbitron’s operations are mainly in the U.S., while Nielsen operates globally. Calhoun said another major driver for the deal is that Nielsen wants to spread Arbitron’s tracking technology to other countries.

Nielsen Holdings N.V. said it will pay $48 per share, which is a 26 percent premium to Arbitron’s Monday closing price of $38.04. Shares of Arbitron, which is based in Columbia, Md., jumped $8.96, or 24 percent, to $47.01 in afternoon trading.

Nielsen, which went public in January 2011, has headquarters in the Netherlands and New York. Its stock added 92 cents, or 3.1 percent, to $30.54 in afternoon trading.

Nielsen said it expects the deal to add about 13 cents per share to its adjusted earnings a year after closing and about 19 cents per share to adjusted earnings two years after closing.

Arbitron’s chief operating officer, Sean Creamer, is set to take over as CEO from William Kerr on Jan. 1. Calhoun said he hoped Creamer would remain with Nielsen after the deal closes.

Nielsen said it has a financing commitment for the transaction.

Nielsen was the prime source of audience ratings in the early days of radio, thanks to a device similar to Arbitron’s People Meter. The Audimeter was attached to the radio set. The company’s focus shifted to TV measurement in the 1950s.